UMTS Femtocells are deployed either on their own 5 MHz frequency sub-band (Downlink-frequency—“UARFCN”), or on a frequency sub-band which they share with UMTS macro-cells (macro base stations). The UARFCN used by femtocells is hereafter referred to as “UARFCN femto.” In most cases, an operator deploying femtocells owns a second (or even third) 5 MHz frequency sub-band that is reserved for exclusive use by macro-cells, which is referred to as “UARFCN macro”.
The operator's objective is to assure that their customers' handsets (“UEs”) re-select from a (large-coverage) marco-cell to a close-by femtocell whenever it makes sense, that is, when the UE has evaluated and confirmed that femtocell is suitable from RF point of view, and that the cell reselection criteria are met. The evaluation is done autonomously by the handset (i.e. without explicit commands from the network) during idle-mode operation. It requires that the UE performs measurements on both the currently serving cell/frequency, as well as on the target-cell/target-UARFCN.